1. Field of the Invention
The present invention relates, in general, to high purity powders, and, more particularly, to high purity fine powders and methods to produce high purity fine powders.
2. Relevant Background
Powders are used in numerous applications. They are the building blocks of electronic, telecommunication, electrical, magnetic, structural, optical, biomedical, chemical, thermal, and consumer goods. On-going market demand for smaller, faster, superior and more portable products has resulted in miniaturization of numerous devices. This, in turn, has demanded miniaturization of the building blocks, i.e. the powders. Sub-micron and nanoscale (or nanosize, ultrafine) powders, with a size 10 to 100 times smaller than conventional micron size powders, enable quality improvement and differentiation of product characteristics at scales currently unachievable by commercially available micron-sized powders.
Nanopowders in particular and sub-micron powders in general are a novel family of materials whose distinguishing feature is that their domain size is so small that size confinement effects become a significant determinant of the materials"" performance. Such confinement effects can, therefore, lead to a wide range of commercially important properties. Nanopowders, therefore, represent an extraordinary opportunity for design, development and commercialization of a wide range of devices and products for various applications. Furthermore, since they represent a whole new family of material precursors where conventional coarse-grain physiochemical mechanisms are not applicable, these materials offer unique combinations of properties that can enable novel and multifunctional components of unmatched performance. Bickmore et al. in U.S. Pat. No. 5,984,997, which along with the references contained therein are hereby incorporated by reference in full, teach some applications of sub-micron and nanoscale powders.
Higher purity materials are needed in electronic applications. For example, silicon is now routinely purified to levels greater than 99.99999% for electronic applications. It is expected that the purity requirements for electronic applications will increase even further. Not only silicon, but other elements from the periodic table and other compounds (metal, semimetal, inorganic or organic) are and will be desired in greater and greater purity. Crystals, bulk materials, fibers, coatings and films are all desired in high purity. Impurities cause failures or defects in electronic and other applications. Higher purity chemicals and materials offer a means of greater product reliability and performance. They also offer means to extend the life of products. For example, batteries prepared from high purity materials offer longer life and superior performance. Existing applications that use commercially available low purity chemicals and materials may all benefit from higher purity chemicals and materials. Since many chemicals and materials are used in the form of powders at some stage, high purity powders are needed and are expected to be needed in the future.
Commonly used high purity powder production techniques are based on starting with commercial grade impure powders and then applying purification techniques to reduce impurities. Some illustrations include leaching, extraction and precipitation, melting, sublimation of volatile impurities, decomposition, chemical reaction, dissolution and crystallization, and electrochemical techniques. These methods are expensive, slow, low volume, and difficult when purities greater than 99.99% are desired. This is one reason why powders with purity greater than 99.9% often enjoy price premiums that are 100 fold higher than readily available low purity powders (95 to 98%).
Padhi and Pillai (U.S. Pat. No. 5,955,052) teach a process which provides high purity lithiated manganese oxide powders, and is hereby incorporated by reference. Their process is a chemical ion exchange reduction method. They do not teach how to reach product purities greater than 99.9%, and their process is expected to be expensive.
Schloh (U.S. Pat. No. 5,711,783) teaches a process for preparing high purity metal powder by reacting one or more volatile alkoxide compounds with a reducing gas, and is hereby incorporated by reference. The process yields a product with very low metal impurities (in the parts-per-million (ppm) range), but with carbon and oxygen impurities. The process is not suitable for production of oxides, carbides, and many other compounds.
Kambara (U.S. Pat. No. 5,722,034, which is hereby incorporated by reference) teaches a method of manufacturing a high-purity refractory metal or alloy using a novel electron beam refining method. This method starts with powders or lumps, but ends up with a sintered material. The method is reported to yield 99.999% pure metal and alloys. This method is anticipated to lead to higher costs. The teachings do not suggest methods for producing high purity inorganics (e.g., oxides). Furthermore, the teachings do not suggest ways to produce high purity powders.
Axelbaum and DuFaux (U.S. Pat. No. 5,498,446, which is hereby incorporated by reference) teach a method and apparatus for reacting sodium vapor with gaseous chlorides in a flame to produce nanoscale particles of un-oxidized metals, composites and ceramics. The invention relates to a development in the production of sub-micron particles and, more particularly, to a development in the flame synthesis of un-agglomerated, nanometer-sized particles of characteristically high purity. The un-oxidized high purity is achieved because of the coating with sodium chloride formed during the flame process. The sodium vapor process is difficult to operate, increases safety concerns, is expensive and is difficult to scale up. It is expected that the powders produced have sodium and chloride contamination arising from the synthesis mechanism used. The teachings are limited to producing particles that are compatible with sodium flame chemistry. Furthermore, the teachings do not specify methods to produce complex materials such as multimetal oxides, carbides, nitrides, borides, and the like.
Krstic (U.S. Pat. No. 5,338,523, and which is hereby incorporated by reference) teaches a process for the production of high purity, high surface area, sub-micron size transition metal carbides and borides. The Krstic method comprises mixing transition metal oxide with carbon in an amount sufficient to form the corresponding carbide or boride. The reactants are heated at a temperature of higher than 1000xc2x0 C. under a small pressure of non-reacting gas and then holding the temperature whilst applying simultaneously sub-atmospheric pressure and agitation until the reaction is complete. Krstic teachings suggest how lower carbon impurities can be achieved over the state of the art, but do not suggest how purities in excess of 99.9% can be achieved. The process is also limited in its economic attractiveness.
Fine powders, as the term used herein, are powders that simultaneously satisfy the following:
1. particles with mean size less than 100 microns, preferably less than 10 microns, and
2. particles with aspect ratio between 100 and 106.
Submicron powders, as the term used herein, are fine powders that simultaneously satisfy the following:
1. particles with mean size less than 1 micron, and 2. particles with aspect ratio between 100 and 106.
Nanopowders (or nanosize or nanoscale powders), as the term used herein, are fine powders that simultaneously satisfy the following:
1. particles with mean size less than 250 nanometers, preferably less than 100 nanometers, and 2. particles with aspect ratio between 100 and 106.
Pure powders, as the term used herein, are powders that have composition purity of at least 99.9%, preferably 99.99% by metal basis.
Powder, as the term used herein encompasses oxides, carbides, nitrides, chalcogenides, metals, alloys, and combinations thereof. The term includes particles that are hollow, dense, porous, semi-porous, coated, uncoated, layered, laminated, simple, complex, dendritic, inorganic, organic, elemental, non-elemental, composite, doped, undoped, spherical, non-spherical, surface functionalized, surface non-functionalized, stoichiometric, and non-stoichiometric form or substance.
Briefly stated, the present invention involves a method of producing metal and alloy fine powders having purity in excess of 99.9%, preferably 99.999%, more preferably 99.99999%. Fine powders produced are of size preferably less than 10 micron, more preferably less than 1 micron, and most preferably less than 100 nanometers. Methods for producing such powders in high volume, low-cost, and reproducible quality are also outlined. The fine powders are useful in various applications such as biomedical, sensor, electronic, electrical, photonic, thermal, piezo, magnetic, catalytic and electrochemical products.